U.S. patent number 6,533,460 [Application Number 10/022,399] was granted by the patent office on 2003-03-18 for hydrodynamic type porous oil-impregnated bearing.
This patent grant is currently assigned to NTN Corporation. Invention is credited to Isao Komori, Natsuhiko Mori, Kazuo Okamura, Makoto Shiranami, Yasuhiro Yamamoto.
United States Patent |
6,533,460 |
Okamura , et al. |
March 18, 2003 |
Hydrodynamic type porous oil-impregnated bearing
Abstract
The porous oil-impregnated bearing 1 comprises a bearing body 1a
made of a porous material, and oil retained in the pores of the
bearing body 1a by impregnation with lubricating oil or lubricating
grease. The inner peripheral surface of the bearing body 1a is
formed with a bearing surface 1b opposed to an outer peripheral
surface of a shaft to be supported, with a bearing clearance
defined therebetween. The bearing surface 1b has a first region m1
in which a plurality of hydrodynamic pressure generating grooves 1c
inclined in one direction with respect to the axial direction are
circumferentially disposed, a second region m2 which is axially
spaced from said first region m1 and in which a plurality of
hydrodynamic pressure generating grooves 1c inclined in the other
direction with respect to the axial direction are circumferentially
disposed, and an annular smooth region n disposed between the first
and second regions m1 and m2.
Inventors: |
Okamura; Kazuo (Mie-ken,
JP), Yamamoto; Yasuhiro (Kuwana, JP),
Komori; Isao (Kuwana, JP), Mori; Natsuhiko
(Mie-ken, JP), Shiranami; Makoto (Inazawa,
JP) |
Assignee: |
NTN Corporation (Osaka-fu,
JP)
|
Family
ID: |
27294460 |
Appl.
No.: |
10/022,399 |
Filed: |
December 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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921602 |
Aug 6, 2001 |
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033651 |
Mar 3, 1998 |
6299356 |
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Foreign Application Priority Data
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Mar 6, 1997 [JP] |
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9-51857 |
Mar 31, 1997 [JP] |
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9-81535 |
Mar 31, 1997 [JP] |
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9-81536 |
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Current U.S.
Class: |
384/114;
384/292 |
Current CPC
Class: |
F16C
33/104 (20130101); F16C 33/145 (20130101); F16C
33/107 (20130101); F16C 17/026 (20130101); F16C
17/107 (20130101); Y10T 29/49639 (20150115) |
Current International
Class: |
F16C
33/10 (20060101); F16C 33/04 (20060101); F16C
017/02 () |
Field of
Search: |
;384/292,114,118,291,107 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2162901 |
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Jun 1973 |
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DE |
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0671268 |
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Apr 1952 |
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GB |
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672268 |
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May 1952 |
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GB |
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2 064 676 |
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Nov 1980 |
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GB |
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2064676 |
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Jun 1981 |
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GB |
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2316453 |
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Feb 1998 |
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GB |
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57/154518 |
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Sep 1982 |
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JP |
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03/071944 |
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Mar 1991 |
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JP |
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070174135 |
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Jul 1995 |
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JP |
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Primary Examiner: Footland; Lenard A.
Attorney, Agent or Firm: Arent Fox Kintner Plotkin &
Kahn
Parent Case Text
This is Division of Application Ser. No. 09/921,602 filed Aug. 6,
2001, which in turn is a Divisional application of parent
application Ser. No. 09/033,651 filed Mar. 3. 1998, now U.S. Pat.
No. 6,299, 356. The disclosure of the prior application(s) is
hereby incorporated by reference herein in its entirety.
Claims
What is claimed is:
1. A hydrodynamic porous oil-impregnated bearing comprising a
porous bearing body formed with bearing surface on an inner
peripheral surface thereof, said bearing surface having inclined
hydrodynamic pressure generating grooves and being opposed through
a bearing clearance to a slide surface of a shaft to be supported
thereby, wherein said bearing flotably supports the slide surface
of the shaft by lubricating oil film formed in the bearing
clearance, and wherein surface openings are distributed on the
bearing surface including the hydrodynamic pressure generating
grooves, and wherein the percentage of area of the surface openings
is not less than 2% but not more than 20%.
2. A hydrodynamic porous oil-impregnated bearing comprising a
porous bearing body formed with bearing surface on an inner
peripheral surface thereof, said bearing surface having inclined
hydrodynamic pressure generating grooves and being opposed through
a bearing clearance to a slide surface of a shaft to be supported
thereby, wherein said bearing floatably supports the slide surface
of the shaft by lubricating oil film formed in the bearing
clearance, and wherein a ratio of a groove depth (h) of the
hydrodynamic pressure generating grooves to the bearing clearance
(c) is within the range;
3. A hydrodynamic porous oil-impregnated bearing as set forth in
claim 1 or wherein said porous blank is formed of a sintered
metal.
4. A hydrodynamic porous oil-impregnated bearing as set forth in
claim 3, wherein said sintered metal contains copper or iron, or
both as a main component.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a hydrodynamic type porous
oil-impregnated bearing being impregnated with lubricating oil or
lubricating grease in a bearing body of porous substance, such as
sintered metal, to have a self-lubricating function, supporting a
slide surface of a shaft in a non-contact manner by a lubricating
oil film produced in a bearing clearance due to hydrodynamic
function of hydrodynamic pressure generating grooves in a bearing
surface. The bearing of the invention is suitable for use
particularly in machines and instruments of which high rotation
accuracy at high speed is required, such as spindle motors for
polygon mirror of laser beam printer (LBP), magnetic disk drives
(HDDs), or the like, and in machines and instruments which are
driven at high speed with a large imbalance load produced in that a
disk is mounted thereon, such as spindle motors for DVD-ROM, or the
like.
In such small-sized spindle motors associated with
information-handling devices, improved rotation performance and
cost reduction are required, as a means therefor, possibility of
changing bearings for the spindle from a rolling bearing to a
porous oil-impregnated bearing has been investigated. However,
since a porous oil-impregnated bearing is a kind of cylindrical
bearing, it tends to produce unstable vibrations where the shaft
eccentricity is small, inducing the so-called whirl in which the
shaft is subjected to a revolving vibration at a rate which is half
the rotary speed. Accordingly, it has heretofore been attempted to
form hydrodynamic pressure generating grooves, such as the
herringbone or spiral shape, in a bearing surface, so as to produce
a lubricating oil film in a bearing clearance by the function of
the hydrodynamic pressure generating grooves which accompanies the
rotation of the shaft, to thereby support the shaft in a noncontact
manner (hydrodynamic type porous oil-impregnated bearing).
A porous oil-impregnated bearing being formed hydrodynamic pressure
generating grooves in a bearing surface is disclosed in Japanese
Utility Model Koukoku Shouwa 63-19627. In this prior art, a region
of the hydrodynamic pressure generating grooves in the bearing
surface is worked to seal surface openings thereon. Such
construction, however, has the following drawback. Since the
surface openings on the region of the hydrodynamic pressure
generating grooves completely sealed, the circulation of oil, which
is the greatest feature of the porous oil-impregnated bearing, is
obstructed. Therefore, the oil which has been exuded in the bearing
clearance is forced into the bent portions of the groove region by
the action of the hydrodynamic pressure generating grooves and
stays there. A great shearing action is present in the bearing
clearance, and this shearing force and frictional heat cause the
oil staying in the groove region to be denatured, while a rise in
temperature tends to accelerate oxidative deterioration of the oil.
Therefore, the bearing life is shortened. On the other hand,
besides plastic processing, it has been proposed to employ coating
or the like as another means for applying a surface treatment,
however, it is necessary that the thickness of such coating film be
less than the groove depth, and it is very difficult to apply a
coating film which is some .mu.m thick solely to the groove
region.
In order to secure the rotation accuracy of the shaft, a plurality
of bearings, e.g., two bearings, are usually used. Further,
bearings are used mostly by being pressed into a housing. Thus, to
secure a substantial alignment of the two bearings, there has been
employed a method in which two bearings are simultaneously pressed
into the housing after a correcting pin is inserted into the
housing. In the case of a bearing having hydrodynamic pressure
generating grooves formed in the bearing surface, if forcible
correction is made by using the correction pin, this will result in
the correction pin cutting into the hydrodynamic pressure
generating grooves in the bearing surface to collapse said grooves,
making it impossible to obtain a stabilized hydrodynamic effect. On
the other hand, the operation of press-fitting without using the
correction pin will fail to provide the necessary alignment between
the bearings. Further, Japanese Patent Kokai Heisei 2-107705
discloses an arrangement in which two bearing surfaces are formed
in axially spaced from each other and in which a region between the
bearing surfaces has a greater diameter than that of the bearing
surfaces. This arrangement, though free from the aforesaid problems
in practice, cannot prevent the unstable vibrations, such as whirl,
because of the lack of hydrodynamic pressure generating grooves in
the bearing surfaces.
As for a method of forming hydrodynamic pressure generating grooves
in bearing surfaces, such a method has been a known that comprises
the steps of inserting into an inner peripheral surface of a
bearing blank a shaft-like jig which holds a plurality of
circumferencially equispaced balls harder than the bearing blank,
imparting a spiral movement to the balls through the rotation and
advance of the jig while pressing the balls against the inner
peripheral surface of the blankm, thereby to plastically work a
region of hydrodynamic pressure generating grooves method of, which
method (Japanese Patent 2541208). In this method, the blank bulges
in a region adjacent the hydrodynamic pressure generating grooves
during forming, and such bulge has to be removed as by lathing or
reaming (Japanese Patent Kokai Heisei 8-232958). For this reason,
the number of manufacturing steps increases. Further, a driving
mechanism and an advancing mechanism for the jig are required, thus
complicating the manufacturing equipment.
SUMMARY OF THE INVENTION
An object of the present invention is to secure the appropriate
circulation of oil between the interior of the bearing body and the
bearing clearance to suppress the deterioration of the oil in the
bearing clearance, thereby increasing the bearing life, and
improving the effect of formation of lubricating oil film in the
bearing clearance, thus increasing the bearing rigidity and
minimizing the shaft deflection due to imbalance load or the
like.
Another object of the invention is to provide an arrangement which
is capable of preventing unstable vibrations such as whirl and
eliminating the inconveniences (such as the loss of shape of
hydrodynamic pressure generating grooves, and axial misalignment)
which are involved in the installing operation.
A further object of the invention is to provide a production method
which facilitates the forming of a bearing surface having inclined
hydrodynamic pressure generating grooves by using simple equipment
and fewer steps and with high accuracy.
To achieve said objects, the invention provides a hydrodynamic type
porous oil-impregnated bearing comprising a porous bearing body
being formed with bearing surface on an inner peripheral surface
thereof, and oil retained in pores of the bearing body by
impregnation of lubricating oil or lubricating grease, wherein the
bearing surface has a first region in which a plurality of
hydrodynamic pressure generating grooves inclined in one direction
with respect to the axial direction are circumferentially disposed,
a second region which is axially spaced from the first region and
in which a plurality of hydrodynamic pressure generating grooves
inclined in the other direction with respect to the axial direction
are circumferentially disposed, and an annular smooth region
positioned between the first and second regions. The bearing
surface of the bearing body is opposed to an outer peripheral
surface of a shaft to be supported, with a bearing clearance
defined therebetween. When a relative rotation occurs between the
bearing body and the shaft, the hydrodynamic pressure generating
grooves mutually reversely disposed in the first and second regions
of the bearing surface cause the oil in the bearing clearance to be
drawn to the annular smooth region and collect in the latter, so
that the oil film pressure in the smooth region is increased. For
this reason, the effect of formation of lubricating oil film is
high. Further, since the smooth region has no groove formed
therein, the bearing rigidity is high as compared with the
construction in which hydrodynamic pressure generating grooves
axially continuous. Therefore, the shaft deflection can be
minimized. Further, it is possible to avoid the lubricating oil
film distribution becoming nonuniform owing to variations in
surface openings on the bearing surface. By the term "surface
openings" is meant those portions of pores of a porous body which
open to an outer surface thereof. In the present invention, the
surface openings are present in the entire region of the bearing
surface including the region formed with the hydrodynamic pressure
generating grooves.
Percentage of area of surface openings in the smooth region of the
bearing surface is preferably smaller than that of the first and
second regions. By the term "percentage of area of surface
openings" is meant the proportion of the total area of the surface
openings in unit area of the outer surface. As a result, since the
oil which is brought together in the smooth region by the
hydrodynamic pressure generating grooves can hardly escape into the
interior of the bearing body through the surface openings on the
smooth region, the capacity of the produced lubricating oil film
can be increased. Further, since an outer peripheral surface of the
shaft is supported in a non-contact manner mainly by the
lubricating oil film formed of the oil collected in the annular
smooth region, the bearing rigidity is high.
The percentage of area of sureface openings is in the range of
5-30%, desirably 5-20%, for the first and second regions and 2-20%,
desirably 2-15%, for the smooth region. If the percentage of area
of surface openings on the first and second regions is less than
5%, the amount of oil to be fed from the interior of the bearing
body to the bearing clearance decreases, resulting in insufficient
formation of lubricating oil film. Reversely, if it exceeds 30%,
the amount of oil which escapes into the interior of the bearing
body becomes excessive, resulting in insufficient formation of
lubricating oil films on the smooth region. Further, if the
percentage of area of surface openings on the smooth region is less
than 2%, the production of the bearing becomes difficult, leading
to an increase in costs. Reversely, if it exceeds 20%, the amount
of oil which escapes into the interior of the bearing body becomes
excessive, resulting in insufficient formation of lubricating oil
film.
In order to enhance the effect of formation of lubricating oil film
on the smooth region, it is preferable that the hydrodynamic
pressure generating grooves in the first region and those in the
second region be symmetric with respect to the axial central region
of the bearing surface.
At the start or stoppage of rotation, the outer peripheral surface
of the shaft comes into instantaneously contact with the bearing
surface of the bearing. At this time, they come into contact with
each other in the axial end region of the bearing surface.
Therefore, by tapering the axial opposite sides of the bearing
surface such that the inner diameter increases toward the bearing
ends (see FIG. 7), the area of their contact is increased when the
apparatus is started or stopped, so that the non-contact state can
be instantaneously established. The first and second regions may be
tapered throughout or portions (associated with the bearing ends)
of each of the first and second regions may be tapered. In
addition, the area of the bearing surface other than the tapered
surface is parallel with the axis.
In this case, the ratio of an increment .DELTA.c in the inner
diameter from the smooth region to the end of the bearing to the
shaft diameter D is .DELTA.c/D=1/3000-1/200, more desirably,
.DELTA.c/D=1/3000-1/500. If .DELTA.c/D is less than 1/3000, the
resulting taper is too small to prevent instantaneous contact, and
if .DELTA.c/D is greater than 1/200, the resulting taper is too
large to provide a useful hydrodynamic effect.
It is possible to provide an arrangement comprising a porous
bearing body being formed with a plurality of axially spaced
bearing surfaces on an inner peripheral surface thereof, at least
one of the plurality of bearing surfaces having inclined
hydrodynamic pressure generating grooves, the inner diameter of the
region between the bearing surfaces being greater than that of the
bearing surfaces, and oil retained in the pores of the bearing body
by impregnation of lubricating oil or lubricating grease. Such
formation of a plurality of bearing surfaces in a single bearing
solves the problem of axial alignment inherent in the case where a
plurality of bearings are incorporated as in the prior art. More
particularly, since the plurality of bearing surfaces are formed in
a single bearing, there is no need to use a correcting pin to
obtain axial alignment as in the case of prior art, and the loss of
shape of the hydrodynamic pressure generating grooves due the use
of such correcting pin does not occur, of course. The formation of
inclined hydrodynamic pressure generating grooves in at least one
bearing surface effectively prevents unstable vibrations such as
whirl.
Provision of a level difference in the boundary between the bearing
surface and the region between the bearing surfaces makes it
possible to effectively reduce the torque loss in the region
between the bearing surfaces.
If the axial section of the region between the bearing surfaces is
drawn with a curve which continuous to the bearing surfaces, oil
which exudes from the surface openings on the region between the
bearing surfaces flows axially along such region, making it easier
to feed the oil to the bearing surface, a fact which means
effective use of oil and enhancement of formation of lubricating
oil film.
The axial section of the region between the bearing surfaces may be
drawn with an arc which is greatest in the middle of the region.
The oil which has exuded from the surface openings on the region
can be easily fed to the bearing surfaces on the opposite
sides.
The outer diameter of an outer portion of the bearing body
corresponding to at least one bearing surface is determined to be
smaller than the outer diameter of an outer portion of the bearing
body corresponding to the region between the bearing surfaces,
whereby when the bearing body is press-fitted in a housing,
deformation of the bearing surfaces under the press-fitting
pressure can be prevented or reduced.
The bearing surface having inclined hydrodynamic pressure
generating grooves can be formed by the following method: the
method comprises the steps of inserting a forming pattern in an
inner peripheral surface of a cylindrical porous blank, the forming
pattern having a first forming portion for forming a region of
hydrodynamic pressure generating grooves and a second forming
portion for forming the other regions in the bearing, applying a
compacting pressure to the porous blank to press the inner
peripheral surface of the porous blank against the forming pattern,
thereby simultaneously forming the region of hydrodynamic pressure
generating grooves and the other region in the bearing surface on
the inner peripheral surface of the porous blank. Alternatively,
disposing the forming pattern in a die, filling powder metal
material between the forming pattern and the die, applying a
compacting pressure to the powder metal material to form a
cylindrical compacted body, while simultaneously forming the region
of hydrodynamic pressure generating grooves and the other region in
the bearing surface on the inner peripheral surface of the
compacted body. Release of the forming pattern can be effected by
utilizing the spring-back of the porous blank due to removal of the
compacting presuure, or by utilizing the spring-back of the
compacted body due to removal of the compacting presuure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view showing an embodiment of a
hydrodynamic type porous oil-impregnated bearing;
FIG. 2 is a longitudinal sectional view conceptually showing a
motor having the hydrodynamic type porous oil-impregnated bearing
of the embodiment;
FIG. 3 is a view schematically showing the flow of oil in the axial
section when a shaft is supported in a non-contact manner by the
hydrodynamic type porous oil-impregnated bearing;
FIG. 4 is a longitudinal sectional view showing another embodiment
comparative of a hydrodynamic type porous oil-impregnated
bearing;
FIG. 5 is a graph showing the results of comparative tests on shaft
deflection when the embodied articles and the comparative article
are used (in the case where the amount of imbalance is small);
FIG. 6 is a graph showing the results of comparative tests on shaft
deflection when the embodied articles and the comparative article
are used (in the case where the amount of imbalance is large);
FIG. 7 is a longitudinal sectional view showing another embodiment
of a hydrodynamic type porous oil-impregnated bearing;
FIG. 8 is a graph showing the results of comparative tests on the
oil film forming state at the start of rotation when the embodied
article and the comparative article are used;
FIG. 9 is a fragmentary enlarged cross sectional view of the
hydrodynamic type porous oil-impregnated bearing;
FIG. 10 is a longitudinal sectional view schematically showing how
the oil is spattered when a shaft is supported in a non-contact
manner by the hydrodynamic type porous oil-impregnated bearing;
FIG. 11 is a longitudinal sectional view showing a sintered metal
blank to be used in an embodiment of the production method;
FIG. 12A is a longitudinal sectional view showing the outline of a
forming device used for forming a bearing surface, and FIG. 12B is
a side view showing a die for forming a bearing surface;
FIGS. 13-15 are views showing the forming steps for a bearing
surface;
FIG. 16 is a graph showing the relation between the inner clearance
and outer interference, and the amount of spring-back;
FIG. 17 is a graph showing the results of comparative tests on
shaft deflection when a cyrindrical bearing and a hydrodynamic type
porous oil-impregnated bearing produced by the production method of
the embodiment are used;
FIG. 18 is a longitudinal sectional view conceptually showing a
testing device used for the comparative tests shown in FIG. 17;
FIG. 19 is a longitudinal sectional view showing an embodiment of a
hydrodynamic type porous oil-impregnated bearing having a plurality
of bearing surfaces;
FIG. 20 is a view schematically showing the flow of oil in the
axial section when a shaft is supported in a non-contact manner by
the hydrodynamic type porous oil-impregnated bearing shown in FIG.
19;
FIG. 21 is a graph showing the relation between the percentage of
area of surface openings on the bearing surface and the kinematic
viscosity of oil;
FIGS. 22 and 24 are graphs showing the results of evaluation tests
on shaft deflection; and
FIG. 23 is a longitudinal sectional view showing another embodiment
of a hydrodynamic type porous oil-impregnated bearing having a
plurality of bearing surfaces.
DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of the present invention will now be described.
FIG. 1 shows by way of example an embodiment of a hydrodynamic type
porous oil-impregnated bearing. This hydrodynamic type porous
oil-impregnated bearing 1 is used, for example, in connection with
a scanner motor for a laser beam printer shown in FIG. 2, to
support a spindle shaft 2 for rotation with respect to a housing 4,
in a non-contact manner, the spindle shaft 2 being rotated at high
speed by magnetic excitation force between a rotor 3 and a
stator.
The porous oil-impregnated bearing 1 comprises a bearing body 1a
made of a porous material, e.g., a sintered metal containing copper
or iron, or both as a main component, and oil retained in the pores
of the bearing body 1a by impregnation with lubricating oil or
lubricating grease. The bearing body preferably contains copper in
20-95 wt %, and has density of 6.4-7.2 g/cm.sup.3.
Tile inner peripheral surface of the bearing body 1a is formed with
a bearing surface 1b opposed to an outer peripheral surface of a
shaft to be supported, with a bearing clearance defined
therebetween, the bearing surface 1b being formed with inclined
hydrodynamic pressure generating grooves 1c. The bearing surface 1b
in this embodiment comprises a first region m1 in which a plurality
of hydrodynamic pressure generating grooves 1c inclined in one
direction with respect to the axial direction are circumferentially
disposed, a second region m2 which is axially spaced from said
first region m1 and in which a plurality of hydrodynamic pressure
generating grooves 1c inclined in the other direction with respect
to the axial direction are circumferentially disposed, and an
annular smooth region n disposed between the first and second
regions m1 and m2. The ribs 1d (the regions between the
hydrodynamic pressure generating grooves 1c) of the first region m1
and the ribs 1d (the regions between the hydrodynamic pressure
generating grooves 1c) of the second region m2 are continuous to
the smooth region n. The hydrodynamic pressure generating grooves
1c of the first region m1 and the hydrodynamic pressure generating
grooves 1c of the second region m2 are symmetrical with respect to
the axial centerline L of the bearing surface 1b. The bearing
surface 1b has surface openings distributed over the entire area
including the region where the hydrodynamic pressure generating
grooves 1c are formed, it being arranged that the oil is circulated
between the interior of the bearing body 1a and the bearing
clearance through the surface openings of the bearing body 1a
including the bearing surface 1b so as to support the outer
peripheral surface of the shaft in a non-contact manner with
respect to the bearing surface 1b. It is advisable that the
percentage of area of surface openings on the smooth region n be
equal or lower than that of the first and second regions m1 and
m2.
When relative rotation takes place between the bearing body 1a and
the shaft, the mutually reversely directed, inclined hydrodynamic
pressure generating grooves 1c formed in the first and second
regions m1 and m2 draw the oil in the bearing clearance toward the
smooth region n, whereby the oil is collected on the smooth region
n; therefore, the oil film pressure on the smooth region n is
increased. Thus, the effect of formation of lubricating oil film is
high. Furthermore, not only the ribs 1d but also the smooth region
n provides a support surface to support the shaft; thus, the area
of support is increased and the bearing rigidity is high. The ratio
r of the axial width of the smooth region n to the bearing width
when the latter is taken to be 1 is preferably in the range of
r=0.1-0.6, more desirably, r=0.2-0.4. If r is less than 0.1 with
respect to the bearing width of 1, the effect to be obtained by
reason of the provision of the smooth region n (improved
hydrodynamic action, and increased bearing rigidity) fails to
manifest itself, whereas if it is greater than 0.6 with respect to
the bearing width of 1, the regions where the hydrodynamic pressure
generating grooves 1c are formed are decreased, exhibiting less
force which urges the oil to the axial central region, thus failing
to develop the hydrodynamic effect. In addition, the hydrodynamic
pressure generating grooves 1c are shown by way of example in a
herringbone form; however, they may be in any form so long as they
are inclined with respect to the axis. For example, they may be in
a spiral form.
FIG. 3 shows the flow of oil 0 in the axial section when the shaft
2 is supported by the porous oil-impregnated bearing 1 of the above
mentioned construction. With the rotation of the shaft 2, the oil 0
retained in the pores of the bearing body 1a exudes from the axial
opposite sides of the bearing surface 1b (and the chamfers) into
the bearing clearance, and is drawn toward the axial center of the
bearing clearance by the hydrodynamic pressure generating grooves.
The pressure of luburicating oil film present in the bearing
clearance is increased by such action of drawing the oil 0 (the
hydrodynamic action). The luburicating oil film formed in the
bearing clearance supports the shaft 2 in a non-contact manner with
respect to the bearing surface 1b without producing unstable
vibrations such as whirl. The oil 0 exuding to flow into the
bearing clearance flows back into the bearing body 1a through the
surface openings on the bearing surface 1b under the pressure
produced with the rotation of the shaft 2, then circulating in the
interior of the bearing body la, again exuding to flow into the
bearing clearance through the surface openings on the bearing
surface 1b (and the chamfers). Generally, since it is difficult to
make uniform the distribution of the surface openings on the
bearing surface, large and small surface openings are present on
the bearing surface. Therefore, the amount of oil which returns to
the interior of the bearing body differs from place to place. As a
result, in the place where oil escapes with ease, oil films hardly
form, whereas in the place where oil hardly escapes, oil films form
with ease, resulting in the oil film in the bearing clearance
having a nonuniform distribution, making it sometimes impossible to
obtain a stabilized hydrodynamic effect. In this connection, the
porous oil-impregnated bearing 1 of this embodiment has the annular
smooth region n between the first and second regions m1 and m2, and
in the smooth region n, the distribution of the surface openings is
easier to uniformly control. Further, in the first and second
regions m1 and m2 the flow of oil in the direction of the grooves
is dominant, while in the smooth region n there is a
circumferential flow of oil, so that even if there are large
surface openings, oil is successively supplied and hence the rate
at which the formation of oil films is reduced is much lower.
The hydrodynamic type porous oil-impregnated bearing 11 shown in
FIG. 4 has a bearing surface 11b being different from the bearing
surface 1b of the above mentioned embodiment in shape. The bearing
surface 11b has a first region in which a plurality of hydrodynamic
pressure generating grooves 11c inclined in one direction with
respect to the axial direction are circumferentially disposed, a
second region which is axially continuous to the first region and
in which a plurality of hydrodynamic pressure generating grooves
11c inclined in the other direction with respect to the axial
direction are circumferentially disposed. The surface openings are
distributed on the entire region of the bearing surface 11b
including regions of the hydrodynamic pressure generating grooves
11c. Under a condition in which there is a little imbalance of a
rotary body so that the bearing rigidity is not required as a
impotant characteristic of a bearing, a bearing surface which has
axially continuous hydrodynamic pressure generating grooves, such
as the above bearing surface 11b, is preferable according to
circumstances.
Various test bearings were incorporated into a small-sized spindle
motor as shown in FIG. 2 and tested for shaft deflection. The
results are shown in FIGS. 5 and 6. FIG. 5 shows the results
obtained when almost no imbalance load is applied (imbalance load:
50 mg.multidot.cm or less), and FIG. 6 shows the results obtained
when large imbalance load is applied (imbalance load: 1
g.multidot.cm). As for the test bearings, use was made of embodied
articles A(.box-solid.) and B(.largecircle.) of the arrangement
shown in FIG. 1, C(.tangle-soliddn.) of the arrangement shown in
FIG. 4, and a cylindrical bearing (a porous oil-impregnated bearing
having no hydrodynamic pressure generating grooves formed in the
bearing surface: (.circle-solid.). The specifications of the test
bearings are as follows. The size of the cylindrical bearing
(.tangle-soliddn.), the size of the bearing clearance and other
specifications than the shape of the bearing surface are the same
as the embodied articles.
[Embodied Article A:.box-solid.] Size:inner dia. .phi.3.times.outer
dia. .phi.6.times.width 3 mm Bearing clearance: 4 .mu.m Percentage
of area of surface openings on bearing surface: 20% Specifications
of hydrodynamic pressure generating grooves Groove depth: 3 .mu.m
Number of grooves: 8 for first region, 8 for second region Angle of
inclination of grooves: 20 degrees Ratio of width of grooves to
width of ribs: 1 Width of bearing surface: 2.4 mm (with 0.3 mm
chamfers on both sides) Width of first and second regions: 0.9 mm
Width of smooth region: 0.6 mm
[Embodied article B: .largecircle.] Size:inner dia.
.phi.3.times.outer dia. .phi.6.times.width 3 mm Bearing clearance:
4 .mu.m Percentage of area of surface openings on bearing surface:
20% for first and second regions, 10% for smooth region
Specifications of hydrodynamic pressure generating grooves Groove
depth: 3 .mu.m Number of grooves: 8 for first region, 8 for second
region Angle of inclination of grooves: 20 degrees Ratio of width
of grooves to width of ribs: 1 Width of bearing surface: 2.4 mm
(with 0.3 mm chamfers on both sides) Width of first and second
regions: 0.9 mm Width of smooth region: 0.6 mm
[Embodied Article C: .tangle-soliddn.] Size: inner dia.
.phi.3.times.outer dia. .phi.6.times.width 3 mm Bearing clearance:
4 .mu.m Percentage of area of surface openings in bearing surface:
20% Specifications of hydrodynamic pressure generating grooves
Groove depth: 3 .mu.m Number of grooves: 8 Angle of inclination of
grooves: 20 degrees Ratio of width of grooves to width of ribs: 1
Width of bearing surface: 2.4 mm (with 0.3 mm chamfers on both
sides)
The embodied article C(.tangle-soliddn.) produced less shaft
deflection than the cylindrical bearing (.circle-solid.) but more
shaft deflection than the embodied articles A, B(.box-solid.,
.largecircle.), and particularly in the region of higher imbalance
load and higher rpm, it produced a large increment in shaft
deflection.
The embodied articles A, B(.box-solid., .largecircle.) produced
less shaft deflection irrespective of the size of the imbalance
load, and particularly in the region of higher rpm, they produced
only a slight increment in shaft deflection. Therefore, the
embodied articles A, B(.box-solid., .largecircle.) can minimize
shaft deflection not only for those devices which are subjected to
low imbalance load, such as LBP motors but also for those devices
which are subjected to high imbalance load when a disk is mounted
thereon, such as DVD-ROM motors.
Next, as shown in FIG. 7, a bearing (an embodied article 2) in
which the axial opposite sides of the bearing surface 1b are
tapered such that the inner diameter was increased toward the
bearing ends and the cylindrical bearing (1) are tested to find the
frequency of contact with the shaft at the start of rotation on the
basis of the oil film-formation percentage. The results are shown
in FIG. 8. In addition, the rpm of the shaft was 6,000.
In the case of the cyrindrical bearing (1), since its oil film
formation percentage at the start of rotation was low, its
frequency of contact with the shaft was high. The reason is that
immediately after the start of rotation, the oil in the bearing
clearance is not affluent and the shaft precesses (swings), so that
at the sides of the bearing surface, the shaft and the bearing
edgewise abut against each other, thus occasioning contact. In
contrast, the embodied article (2) had undergone no contact with
the shaft since the rotation started and instead an oil film was
instantly formed therein. The reason is that since the axial
opposite sides of the bearing surface 1b are tapered, the edgewise
abutment between the shaft and bearing is avoided.
In addition, there is an optimum range in the ratio of the
hydrodynamic pressure generating groove depth to the radial
clearance, outside which range the hydrodynamic effect is greatly
reduced. If c/h is in the range of 0.5-5.0 (see FIG. 9), a high
rotation accuracy which causes no problems in practice can be
maintained.
Further, although porous oil-impregnated bearings are usually used
without being fed with oil, gradual exhaustion or outflow of the
internally retained oil due to spattering and evaporation of the
oil cannot be avoided. When the oil has been exhausted, the range
of oil film formation decreases, leading to degradation of the
rotation accuracy, such as shaft deflection. Particularly, a shaft
is used often in its vertical position, as shown in FIG. 10, and in
the case of a laser beam printer motor which is used at a high
speed of 10,000 rpm, the oil retained internally of the bearing
tends to flow out under the action of centrifugal force, so that it
has been difficult to maintain the performance, such as the
formation of oil films. In the case of LB and HDD, discontinuation
of oil films is fatal to the maintenance of high rotation accuracy.
In the case of a single porous oil-impregnated bearing,
particularly when the shaft is rotated at high speed, the oil,
taking in the ambient air, is circulated in the bearing, sometimes
resulting in the air migrating into the bearing clearance. To
prevent the migration of air, it is effective to place an oil
re-feeding member in close contact with the bearing body, so as to
re-feed oil from the oil re-feeding member as soon as even very few
empty pores are created. Placement of an oil re-feeding member
brings about not only the effect of prolonging life but also the
effect of maintaining an oil film which is necessary for
maintaining high accuracy. The oil re-feeding member used in close
contact with the bearing body may be in the known form of a porous
body, such as metal or resin, or a fibrous material, such as felt,
impregnated with oil, but it is preferable to use a solid
lubricating composition which has the nature of gradually
continuously exuding the internally retained oil to the surface at
temperatures of at least 20C. It is recommendable to use, e.g., a
solid resin lubricating composition prepared by melting a mixture
of lubricating oil or lubricating grease and superhigh molecular
weight polyethylene powder, and cooling the melt to solidify the
latter. This solid resin lubricating composition continuously
exudes the retained oil at not less than ordinary temperatures,
making it possible to continuously re-feed oil to the bearing.
Further, this solid resin lubricating composition can be
mass-produced at low cost and is easy to handle.
Thus, if a solid resin lubricating composition which gradually
continuously exudes oil to the surface even when left to stand at
not less than ordinary temperatures is placed in close contact with
the surface of the bearing, then even if the oil in the bearing
flows away, oil is re-fed into the interior of the bearing by the
capillary action which occurs in the pores of the bearing body, so
that a satisfactory hydrodynamic oil film can be formed at all
times. This solid resin lubricating composition can be produced by
the following method.
For example, it is obtained by uniformly mixing a predetermined
amount of lubricating grease or lubricating oil with a
predetermined amount of superhigh molecular weight polyolefin
powder, pouring the mixture into a die of predetermined shape, and
melting the mixture at temperatures not less than the gelling
temperature of the superhigh molecular weight polyolefin powder and
not more than the dropping point of lubricating grease if such
grease is used, and cooling the mixture at ordinary temperatures.
The superhigh molecular weight polyolefin powder may be a powder of
polyethylene, polypropylene, or polybutene or a copolymer thereof,
or a mixture of these powders, the molecular weight of each powder
being so selected that the average molecular weight measured by the
viscosity method is 1.times.10.sup.6 -5.times.10.sup.6. Polyolefins
which are within the range of such average molecular weight are
superior to low molecular weight polyolefins in rigidity and oil
retention and will hardly flow even heated to high temperatures.
The proportion of such superhigh molecular weight polyolefin in the
lubricating composition is 95-1 wt %, and the amount depends on the
desired degree of bleeding, toughness and hardness of the
composition. Therefore, the greater the amount of superhigh
molecular weight polyolefin, the higher the hardness of the gel
after dispersion at a predetermined temperature.
Further, the lubricating grease used in this invention is not
particularly restricted, and may be a soap-thickened or
non-soap-thickened lubricating grease, examples of such lubricating
grease being lithium soap-diester type, lithium soap-mineral oil
type, sodium soap-mineral oil type, aluminum soap-mineral oil type,
lithium soap-diester mineral oil type, non-soap-diester type,
non-soap-mineral oil type, non-soap-polyolester type, and lithium
soap-polyolester type. The lubricating oil is not particularly
restricted, either, examples thereof being diester type, mineral
oil type, diester mineral oil type, polyolester type, and
polyaolefin type. In addition, the base oil for the lubricating
grease or the lubricating oil is desirably the same lubricating oil
as that with which the porous oil-impregnated bearing is initially
impregnated, but it may be more or less different therefrom so long
as the lubricating characteristics are not impaired.
Although the melting points of the superhigh molecular weight
polyolefins mentioned above are not constant as they vary according
to their respective average molecular weights, one, e.g., having an
average molecular weight of 2.times.10.sup.6 as measured by the
viscosity method has a melting point of 136.degree. C. As for a
commercially available one having the same average molecular
weight, there is Mipelon (registered trade mark) XM-220, produced
by Mitsui Petrochemical Industries, Ltd., and the like.
Therefore, when it is desired to disperse superhigh molecular
weight polyolefin in the aforesaid lubricating grease or
lubricating oil and retain it therein, said materials, after being
mixed, are heated to a temperature not less than the gelling
temperature of the superhigh molecular weight polyolefin and if
lubricating grease is used, to a temperature less than the dropping
point thereof, e.g., to 150-200.degree. C.
Such bearing device can be widely utilized, for example, in various
motors, including laser beam printer polygon mirror motors,
magnetic disk drive spindle motors, and DVD-ROM motors, and motors
for axial fans, ventilating fans, electric fans and other electric
appliances, electric parts for cars, etc, and their durability can
be greatly improved by hydrodynamically supporting the shaft.
The bearing body 1a of the porous oil-impregnated bearing 1 shown
in FIG. 1 can be produced by compacting a metal powder material
contains copper or iron, or both as a main component, sintering it
to obtain a cylindrical sintered metal blank 13 shown in FIG. 11,
and subjecting said blank to sizing.fwdarw.rotation
sizing.fwdarw.bearing surface forming.
The sizing process is a process for sizing the outer and inner
peripheral surfaces of the sintered metal blank 13, which is
performed by press-fitting the outer peripheral surface of the
sintered metal blank 13 in a cylindrical die while press-fitting a
sizing pin in the inner peripheral surface. The rotation sizing
process is a process in which a polygonal sizing pin is
press-fitted in the inner peripheral surface of the sintered metal
blank 13 and then the inner peripheral surface is sized while the
sizing pin is rotated. The bearing surface forming process is a
process in which a forming pattern having a shape corresponding to
the bearing surface 1b of a finished product 1a is pressed against
the inner peripheral surface of the sintered metal blank 13 having
said sizing treatment applied thereto to thereby simultaneously
form a region of hydrodynamic pressure generating grooves 1c and
the other regions (ribs 1d and annular smooth region n) in the
bearing surface 1b. This process is, for example, as follows.
FIG. 12A shows by way of example the outline construction of a
forming machine used in the bearing surface forming process. This
device comprises a cylindrical die 20 in which the outer peripheral
surface of the sintered metal blank 13 is to be press-fitted, a
core rod 21 for forming the inner peripheral surface of the
sintered metal blank 13, and upper and lower punches 22 and 23 for
holding the upper and lower end surfaces of the sintered metal
blank 13. As shown in FIG. 12B, the outer peripheral surface of the
core rod 21 is formed with forming pattern 21a in concave-convex
form corresponding to the shape of the bearing surface 1b of a
finished product. The convex portion 21a1 of the forming pattern
21a is to form the region of the hydrodynamic pressure generating
grooves 1c in the bearing surface 1b, while the concave portion
21a2 is to form the other region (ribs 1d and annular smooth region
n) than the region of the hydrodynamic pressure generating grooves
1c in the bearing surface 1b. The level difference (depth H, for
example 2-5 .mu.m) between the convex and concave portions 21a1 and
21a2 of the forming pattern 21a is as deep as the hydrodynamic
pressure generating grooves 1c in the bearing surface 1b, but it is
shown considerably exaggerated in the figure.
Before the sintered metal blank 13 is press-fitted in the die 20,
there is an inner clearance T between the inner peripheral surface
of the sintered metal blank 13 and the forming pattern 21a of the
core rod 21 (based on the convex portion 21a1). The size
(diametrical value) of the inner clearance T is, e.g., 50 .mu.m.
The press-fit allowance (outer interference U: diametrical value)
for the outer peripheral surface of the sintered metal blank 13
with respect to the die 20 is, e.g., 150 .mu.m.
After the sintered metal blank 13 is placed on the die 20 for
alignment, as shown in FIG. 13, the upper punch 22 and core rod 21
are lowered to press-fit the sintered metal blank 13 in the die 20
to urge it against the lower punch 23, thereby pressing it from
above and below.
The sintered metal blank 13 receives a compacting pressure from the
die 20 and upper and lower punches 22, 23 and is thereby deformed,
with the inner peripheral surface thereof pressed against the
forming pattern 21a of the core rod 21. The amount of compression
of the inner peripheral surface of the sintered metal blank 13 is
approximately equal to the difference between the outer
interference U and the inner clearance T, and the surface layer
portion of the sintered metal blank 13 extending from the inner
peripheral surface to a predetermined depth is pressed by the
forming pattern 21a of the core rod 21, producing a plastic flow
which cuts into the forming pattern 21a. Thereby, the shape of the
forming pattern 21a is transferred to the inner peripheral surface
of the sintered metal blank 13, whereby the bearing surface 1b is
formed to have the shape shown in FIG. 1.
After the forming of the bearing surface 1b is completed, as shown
in FIG. 14, with the core rod 21 inserted in the sintered metal
blank 13, the lower punch 23 and core rod 21 are operatively lifted
(the state of FIG. 14 2) and the sintered metal blank 13 is
extracted from the die 20 (the state of FIG. 14 1). When the
sintered metal blank 13 is extracted from the die 20, an amount of
spring-back Q is produced in the sintered metal blank 13 to
increase the inner diameter of the latter (see FIG. 15), so that
the core rod 21 can be extracted from the inner peripheral surface
of the sintered metal blank 13 without breaking the hydrodynamic
pressure generating grooves 1c (the state of FIG. 14 4). This
completes the bearing body 1a.
FIG. 16 shows the relation between the inner clearance T and outer
interference U and the amount of spring-back Q when said bearing
surface forming process has been performed on a sintered metal
blank of inner diameter .phi.3, outer diameter .phi.6 and width 3
mm. As shown in this figure there is a certain interrelation
between the inner clearance T and outer interference U and the
amount of spring-back Q, it being understood that when the inner
clearance T and outer interference U are specified, the amount of
spring-back Q is specified. According to experiments, it has been
found that at a predetermined groove depth H (2-3 .mu.m), if the
amount of spring-back Q is set at 4-5 .mu.m (diametrical value),
the sintered metal blank 13 can be extracted from the core rod 21
without breaking the hydrodynamic pressure generating grooves 1c;
thus, it is advisable to set the inner clearance T and outer
interference U in such a manner as to provide the amount of
spring-back Q to that degree. In addition, when the radial amount
of the spring-back Q of the sintered metal blank 13 is greater than
the depth H of the hydrodynamic pressure generating grooves 1c, the
forming pattern 21a can be released without interfering with the
inner peripheral surface of the sintered metal blank 13. However,
even when the radial amount of the spring-back Q of the sintered
metal blank 13 is less than the depth H of the hydrodynamic
pressure generating grooves 1c and the forming pattern 21a more or
less interferes with the inner peripheral surface of the sintered
metal blank 13, it may be enough when the forming pattern 21a can
be released from the inner peripheral surface of the sintered metal
blank 13 without breaking the hydrodynamic pressure generating
grooves 1c, with adding an increase in diameter (radial amount) of
the sintered metal blank 13 due to the material elasticity of the
sintered metal blank 13.
In addition, after the forming process for the bearing surface 1b
has been completed, the bearing surface 1b may be sized by using an
ordinary sizing pin (of circular cross section). In this case, the
ribs 1d and smooth region n in the bearing surface 1b are sized by
the sizing pin, whereby the percentage of area of surface openings
on their region becomes lower than that of the region of the
hydrodynamic pressure generating grooves 1c. Also, such a forming
process for the bearing surface may be emploied that comprising the
steps of forming only the regin of the hydrodynamic pressure
generating grooves by the forming pattern, and then sizing or
rotation sizing the other region in the bearing surface.
The bearing body 1a is produced through the processes described
above and is impregnated with lubricating oil or lubricating grease
to retain oil, whereupon the hydrodynamic type porous
oil-impregnated bearing 1 in the form shown in FIG. 1 is
completed.
Comparative tests for shaft deflection were conducted using
cylindrical bearing (a porous oil-impregnated bearings having no
hydrodynamic pressure generating grooves formed in the bearing
surface) and hydrodynamic type porous oil-impregnated bearings
produced by the aforesaid method. The tests were conducted by
incorporating test bearings in CD-ROM motors shown in FIG. 18, with
a commercially available CD set therein, the shaft deflection
relative to rpm was measured. The results are shown in FIG. 17. It
is seen from this figure that as compared with cylindrical bearing,
the hydrodynamic type porous oil-impregnated bearings of the
embodiment are effective in suppressing shaft deflection.
In the above embodiment, the forming process for the bearing
surface has been applied to the sintered metal blank 13; however,
it may be performed in a compacting process for powder metal
material. This compacting process is such a process that comprises
the steps of disposing a forming pin in a die, filling the powder
metal material between the forming pin and the die, applying a
compacting pressure to the powder metal material to form into a
cylindrical form. In this compacting process, it is possible to
form a bearing surface as shown in FIG. 1 at the same time of
compacting a compacted body, by being provided with forming
pattern, as shown in FIG. 12B, on the outer peripheral surface of
the forming pin. Further, after compaction, the compacted body can
be released from the forming pin while utilizing the spring-back of
the compacted body due to removal of the compacting pressure,
without any possibility of the bearing surface losing its shape.
The compacted body is sintered, and then it is finished through
sizing, impregnation with oil, etc.
In addition, it is only necessary that the bearing body be porous;
thus, it is not limited to said sintered metal but may, e.g., be a
porous body formed by foaming. As blanks therefor, cast iron,
synthetic resin, ceramics and the like may be used. Further, in the
above embodiment, the spring-back of the formed body has been
utilized for releasing the forming pattern; however, the forming
pattern may be constructed such that it can be elastically
decreased in diameter. Thus, after the forming of the bearing
surface, the forming pattern may be elastically decreased in
diameter to be released from the formed product. Futher, when
forming the bearing surface 11b shown in FIG. 4, the forming
pattern may be shaped as corresponding to the shape of the bearing
surface 11b.
FIG. 19 shows the state in which a hydrodynamic type porous
oil-impregnated bearing 1' having a plurality of bearing surfaces
1b' is fixed to a housing 5. The porous oil-impregnated bearing 1'
comprises a porous body, e.g., a bearing body 1a' of sintered metal
containing copper or iron, or both as a main component and oil
retained in the pores of the bearing body 1a' by impregnation with
lubricating oil or lubricating grease.
The inner peripheral surface of the bearing body 1a' is formed with
a plurality of, for example, two, axially spaced bearing surfaces
1b' opposed to an outer peripheral surface of a shaft to be
supported, each of the two bearing sufaces 1b' being formed with a
plurality of circumferentially disposed hydrodynamic pressure
generating grooves 1c'. In the same way as shown in FIG. 4, the
hydrodynamic pressure generating grooves 1c' in this embodiment
have a V-shaped continuous form having a pair of groove regions,
with the grooves in one region inclined in one direction with
respect to the axial direction and the grooves in the other region
inclined in the other direction with respect to the axial
direction. The surface openings are distributed on both regions of
the hydrodynamic pressure generating grooves 1c' and ribs 1e' in
the bearing surfaces 1b'. In addition, it is sufficient to form the
hydrodynamic pressure generating grooves 1c' in at least one of the
bearing surfaces 1b'.
The region 1d' between the bearing surfaces 1b' of the bearing body
1a' has an inner diameter D1 which is greater than the inner
diameter D2 of the bearing surfaces 1b' {strictly, the inner
diameter of the region of the ribs 1e' (corresponding to 1d in FIG.
9) between the hydrodynamic pressure generating grooves 1c' }. In
this embodiment, the axial section of the region 1d' is described
with a single arc continuous to the bearing surfaces 1b', the
largest diameter portion of said arc being located at the axial
center of the region 1d'. In addition, level differences may be
provided in the boundaries between the region 1d' and the bearing
surfaces 1b'. Further, the axial section of the region 1d' may be
described with other curves, besides an arc, such as ellipse,
parabola, etc. It may be described with a combination of two like
curves (for example, two arcs), a combination of two dissimilar
curves (for example, an arc and parabola) or a combination of a
curve and a straight line. The largest diameter portion of the
region 1d' may be deviated to the side associated with one bearing
surface 1b'.
Further, in this embodiment, the outer diameter D3 of the outer
portions 1f' corresponding to the two bearing surfaces 1b' is
smaller than the outer diameter D4 of the outer portion 1g'
corresponding to the region 1d' between the bearing surfaces 1b' in
the bearing body 1a'. When the porous oil-impregnated bearing 1' is
press-fitted in the inner periphery of a housing 5 in the manner
shown in the figure, deformation of the bearing surfaces 1b' due to
the fitting force can be prevented or mitigated, so that
substantial accuracy can be obtained. The fixing force can be
obtained through the interference between the outer portion 1g' and
the housing 5. The region 1d' is larger in diameter than the
bearing surfaces 1b' and does not take part in supporting the
shaft, so that even if an amount of deformation corresponding to
the fitting force takes place, there is no influence on the
accuracy of the bearing. The difference between the outer diameter
D3 of the outer portions 1f' and the outer diameter D4 of the outer
portion 1g' (the difference before press-fitting) is determined
such that in consideration of the interference with the housing 5
(the interference of the outer portion 1g'), the outer portion 1f'
does not contact the inner periphery of the housing 5 or provides
an amount of interference which does not influence the bearing
accuracy. In addition, the outer diameter of only one of the two
outer portions 1f' may be determined in the manner described
above.
FIG. 20 shows the flow of oil in an axial section when the shaft 2
is supported by the porous oil-impregnated bearing 1' arranged in
the manner described above. As the shaft 2 is rotated, the oil 0
retained in the bearing body 1a' exudes from the axial opposite
sides of each bearing surface 1b' to enter the bearing clearance
and then it is drawn to the axial center of the bearing clearance
by the hydrodynamic pressure generating grooves. The action of
drawing the oil 0 (hydrodynamic action) increases the pressure of
the oil film present in the bearing clearance, thus forming a
lubricating oil film. This lubricating oil film formed in the
bearing clearance supports the shaft 2 in a non-contact manner with
respect to the bearing surfaces 1b' without causing unstable
vibrations such as whirl. The oil 0 exuding into the bearing
clearance returns to the interior of the bearing body 1a' through
the surface openings in the bearing surfaces 1b' under the action
of the generated pressure which accompanies the rotation of the
shaft 2, the oil circulating in the interior of the bearing body
1a' and again exuding into the bearing clearance through the
bearing surfaces 1b'. In this way, the oil 0 retained in the
bearing body 1a' continuously supports the shaft 2 in a non-contact
manner by the hydrodynamic effect while circulating between the
bearing clearance and the bearing body 1a'.
Since this porous oil-impregnated bearing 1' supports the shaft 2
in a non-contact manner by the two axially spaced bearing surfaces
1b', the shaft 2 can be accurately supported by one bearing.
Further, the drawing action of the hydrodynamic pressure generating
grooves 1c' produces a negative pressure in the space defined
between the region 1d' between the bearing surfaces 1b' and the
outer peripheral surface of the shaft 2 and the oil 0 exudes also
from the surface openings on the region 1d' and is fed to the
bearing surfaces 1b', thereby enhancing the formation of
lubricating oil film in the bearing clearance and increasing the
bearing rigidity. Particularly, in the case where the axial section
of the region 1d' is described with an arc (or other curve)
continuous to the bearing surfaces 1b' as in this embodiment, the
oil 0 exuding from the surface openings on the region 1d' flows
axially along the region 1d' until it is effectively fed to the
bearing surfaces 1b', a fact which leads to the effective use of
oil and the enhancement of formation of lubricating oil film.
In order to keep such circulation of oil satisfactory, it is
desirable that the surface openings be substantially uniformly
distributed on both regions of the hydrodynamic pressure generating
grooves 1c' and ribs 1e' in the bearing surfaces 1b'. If the
proportion of the surface openings (the percentage of area of
surface openings) in the surface is decreased, the oil becomes less
mobile and reversely if it is increased, the oil becomes more
mobile. Further, the viscosity of oil is related to the mobility of
oil such that if the viscosity is low, the mobility is high and if
it is high, the mobility is low.
If the percentage of area of surface openings is high and the
viscosity is low, the oil becomes extremely mobile but the oil
exuded into the bearing clearance is readily returned to the
interior of the bearing body by the action of the hydrodynamic
pressure generating grooves, thereby decreasing the hydrodynamic
effect. Reversely, if the percentage of area of surface openings is
low and the viscosity is high, the oil becomes extremely immobile,
so that, though the pressure of the lubricating oil film increases,
the proper circulation of oil is impeded and the degradation of oil
is accelerated.
Therefore, there is an optimum range between the percentage of area
of surface openings and the viscosity of oil which secures the
formation of lubricating oil film necessary for supporting the
shaft in a non-contact manner and which also secures the
appropriate circulation of oil.
To clarify this optimum range, evaluation tests were conducted by
using LBP motors. The LBP motors used in the evaluation tests had a
shaft diameter of .phi.4 and a mirror installed therein, the rpm
being 10,000, the surrounding temperature being 40.degree. C. The
results are shown in FIG. 21. In this figure, ".largecircle."
indicates the absence of problems in 1,000-hour continuous running
endurance test. And ".DELTA." indicates that troubles occurred,
during 500-1,000 hours, such as an increase in shaft deflection (5
.mu.m or above), an increase in torque=a decrease in rpm (the rpm
failed to increase to 10,000 rpm) and abnormal sound and that
normal operation was impossible. The mark "X" indicates that such
troubles occurred within 500-1,000 hours.
It is seen from the above evaluation tests that the optimum range
of the percentage of area of surface openings and the oil viscosity
(the region where there is no "X") is the area surrounded by solid
line in FIG. 21, which area satisfies the following conditions: a)
The percentage of area of sufade openings on the bearing surface
including the region of the hydrodynamic pressure generating
grooves is not less than 2% but not more than 20%; b) The kinematic
viscosity of retained oil at 40.degree. C. is not less than 2 cSt;
c) The percentage of area of surface openings on the bearing
surface and the kinematic viscosity of oil at 40.degree. C. satisfy
the relation
where A; percentage of area of surface openings [%] .eta.;
kinematic viscosity of oil at 40.degree. C. [cSt]
Selecting the percentage of area of surface openings and the oil
viscosity within such range ensures formation of a sufficient
lubricating oil film to support the shaft in a non-contact manner
and its proper circulation, so that high rotation accuracy and long
life can be attained.
There is an optimum range of ratio of the depth (h) of the
hydrodynamic pressure generating grooves to the size of the bearing
clearance (radial clearance: c) and it is believed that with values
outside the range, the sufficient hydrodynamic effect cannot be
obtained. To clarify this optimum range, evaluation tests were
conducted by replacing the shaft of the LBP motor by a longer one
to allow measurement of shaft deflection. The rpm was 10,000 and
the test ambient atmosphere was at ordinary temperatures and
humidity, and the LBP motor was .phi.4, and did not have a mirror
installed therein. In addition, the shaft defection was measured
with a non-contact type displacement gauge.
Under the above conditions, values of the shaft deflection relative
to the c/h (c; radial clearance, h; groove depth) were plotted, and
the results shown in FIG. 22 were obtained. It is seen from FIG. 22
that when the c/h is in the range of 0.5-4.0, then the shaft
deflection is not more than 5 .mu.m, but if it is less than 0.5 or
greater than 4.0, then the shaft deflection is not less than 5
.mu.m. Therefore, to maintain high accuracy, it is desirable that
the c/h be in the range of 0.5-4.0. In addition, it is desirable
that the size c of the bearing clearance (radial clearance) be such
that when the radius of the shaft is R, then the c/R is in the
range of 1/2,000-1/400.
A hydrodynamic type porous oil-impregnated bearing 1" shown in FIG.
23 also has a plurality of bearing surfaces; however, the shape of
the bearing surfaces differs from that of the hydrodynamic type
porous oil-impregnated bearing 1' shown in FIG. 19.
Each of the bearing surfaces 1l" of the porous oil-impregnated
bearing 1" in this embodiment comprises a first region m1 in which
a plurality of hydrodynamic pressure generating grooves 1c1
inclined in one direction with respect to the axial direction are
circumferentially disposed, a second region m2 which is axially
spaced from said first region m1 and in which a plurality of
hydrodynamic pressure generating grooves 1c2 inclined in the other
direction with respect to the axial direction are circumferentially
disposed, and an annular smooth region n disposed between the first
and second regions m1 and m2. The ribs 1e1 of the first region m1
and the ribs 1e2 of the second region m2 continuous to the smooth
region n. When a relative rotation is produced between the bearing
body 1a" and the shaft, the hydrodynamic pressure generating
grooves 1c1 and 1c2 formed in the first and second regions m1 and
m2 in a mutually reversely inclined manner draw oil into the smooth
region n to collect the oil in the smooth region n, whereby the oil
film pressure in the smooth region n is increased. Furthermore,
since the smooth region n has no grooves formed therein, the effect
of formation of lubricating oil film in this region is high, and in
addition to the ribs 1e1 and 1e2, the smooth region n provides a
support surface for supporting the shaft, whereby the support area
is increased and so is the bearing rigidity. Further, the axial
section of the region 1d" between the bearing surfaces 1b" is
described with an axial straight line, and the boundaries between
the region 1d" and the bearing surfaces 1b" form level differences
1h. In addition, the axial section of the region 1d" may be
described with a combination of two straight lines inclined with
respect to the axial direction (V-shaped type).
In addition, as in the case of the hydrodynamic type porous
oil-impregnated bearing 1' shown in FIG. 19, the inner diameter of
the region 1d" is greater than that of the bearing surfaces 1b",
and the outer diameter of the outer portions 1f" corresponding to
the bearing surfaces 1b" is smaller than that of the outer portion
1g" corresponding to the region 1d".
Comparative tests on press-fitting in a housing and rotation
accuracy comparative tests were conducted. The results are
described below.
(1) Comparative tests on Press-fitting in Housing
Comparative article: Constructed such that it has a single bearing
surface having hydrodynamic pressure generating grooves formed
therein. Two test bearings were produced, whose inner diameter
before press-fitting was .phi.3.006, and they were press-fitted in
a housing with an interference of 18 .mu.m, the correcting pin
diameter being .phi.3.000 mm.
Embodied article: Constructed such that it has two bearing surface
each having hydrodynamic pressure generating grooves formed
therein. The test bearing was press-fitted in a housing under the
same conditions as above.
Test results: In the case of the comparative article, the two
bearings had part of their hydrodynamic pressure generating grooves
collapsed. The tests were conducted with the bearings installed in
motors, and the rotation was unstable, producing a shaft deflection
and the like which are worse than in the case of ordinary
cyrindrical bearings (bearings which have no hydrodynamic pressure
generating grooves formed in their bearing surfaces). The cause of
collapse of part of the hydrodynamic pressure generating grooves
seems to be the local thickening of material in the test bearings
(same with bearing products); therefore, it is believed that the
correcting force from the correcting pin acted heavily on part of
the hydrodynamic pressure generating grooves. In contrast thereto,
in the embodied article, although the groove depth was found
decreased as a whole (from 4 .mu.m to 3.5 .mu.m), there was
observed no phenomenon in which part thereof was collapsed. When
the bearing was installed in a motor and the shaft deflection was
measured, it exhibited an excellent performance; the shaft
deflection was not more than 2 .mu.m at 2,000-15,000 rpm.
(2) Rotation Accuracy Comparative Tests
Comparative article: Constructed such that it has two bearing
surfaces each having no hydrodynamic pressure generating grooves
formed therein.
Embodied article: Constructed such that it has two bearing surfaces
each having hydrodynamic pressure generating grooves formed therein
(the construction being shown in FIG. 19).
Test results: The test results are shown in FIG. 24. As shown in
this figure, the embodied article, as compared with the comparative
article, exhibited a superior performance {the mark ( .box-solid.)
indicates measured data for the embodied article and
(.circle-solid.) for the comparative article).
In addition, the hydrodynamic type porous oil-impregnated bearing
having a plurality of bearing surfaces can be produced by the
aforesaid method using a core rod or forming pin in which forming
patterns corresponding to the shape of the bearing surfaces are
formed in a plurality of places on the outer peripheral surface
thereof.
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